Folded transducer transmitting or receiving for low frequency underwater sound



June 10, 1969 w. L. ANGELOFF 3,449,712

FOLDED TRANSDUCER TRANSMITTING OR RECEIVING FOR LOW FREQUENCY UNDERWATER SOUND Filed Oct. 31, 1967 FIG.

INVENTOR.

WESLEY L. ANGELOFF BY new. cum/53.,

A TTORNEYS United States Patent O 3,449,712 FOLDED TRANSDUCER TRANSMITTING OR RECEIVING FOR LOW FREQUENCY UNDER- WATER SOUND Wesley L. Angelofl, San Diego, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Oct. 31, 1967, Ser. No. 679,299 Int. Cl. H04h 13/02 U.S. Cl. 340-8 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to transducers of acoustic energy transmitted or received through a water medium. The invention includes a stack of electromechanical elements such as stacked piezo-electric cylinder, that are deformable in accordance to an impressed electric field when the transducer is used as a transmitter or that yield an electric output signal representative of a mechanical deformation of the stack in accordance with impinging acoustic energy. A concentric housing extends inwardly from opposite ends of the stack to terminate in a pair of radially, outwardly extending, flanged, horn-shaped surfaces peripherally joined by a resilient annular ring. Thus arranged, the surfaces are free to vibrate in accordance with impinging acoustic energy or in response to electric potentials impressed across the electromechanical stack. Relevant acoustic transducer theory dictates that the resonant frequency of the present type of transducer is dependent upon the length of the stacks, the mass of the stack and of the radiating or receiving surface. By folding back a light weight housing attached at opposite ends of the electromechanical stack to concentrically enclose the stack to terminate in a radiating-receiving surface, the present transducer has effectively increased the characteristics of the transducer stack to give an optimum low frequency response. By being so constructed an extremely compact transducer exists considering the low frequency at which it operates.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The invention pertains to low frequency acoustic transducers. Current low frequency transducers of acoustic energy are large and unmanageable in an operational environment due to the fact that low frequency operation depends in part on the physical configuration of the transducer itself. No existing transducers utilize the linkage from the electromechanical elements to the radiating surface as the housing for the transducer. Nor are there in the prior art radiating surfaces in a substantially opposed relationship adjoined only at the outer peripheral edges by a resilient element. This arrangement in the instant transducer permits a compactness not realized in existing transducers.

SUMMARY OF THE INVENTION The present invention is directed to providing a compact transmitter-receiver transducer which is responsive to signals representative of acoustic energy. The transducer ice includes an axially extending electromechanical drive element which axially displaces a pair of axial faces in accordance with the signals. The pair of housing elements being tubular in shape and concentrically arranged around the electromechanical drive element are attached at one end to the axial faces, extend inwardly over the drive element, and terminate in a pair of radially extending horn shaped diaphragm surfaces. The surfaces joined by an annular resilient ring disposed therebetween in a peripheral relationship to permit axial motion of the diaphragm surfaces in accordance with impinging acoustic energy or in accordance with the actual deformation of the electromechanical drive element by an impressed potential. The continuity between the electromechanical driving element, the housing element, and the diaphragm surfaces permits a relatively light transducer mass and a relatively long, additive length which are both directly proportional to the lowest resonant frequency of the transducer. Such low frequency operation is not attained at a cost of great weight and bulk since the lightweight housing additionally serves as a linkage from the electromechanical drive element to the radiating surfaces.

It is an object of the present invention to provide the transducer capable of sustained low frequency operation which is compact at a relatively high efliciency.

It is another object of the instant invention to provide a transducer capable of operating over a relatively broad band.

It is a further object of the invention to provide a transducer which is rugged and ideally adaptable to environments encountering adverse operational conditions.

Still another object of the invention is to provide an omnidirectional transducer for radiation of broad band acoustic energy.

These and other objects of the invention will become more readily apparent from the ensuing descriptions, drawings and claims.

FIG. 1 shows the transducer in an operational environment supported by a yolk from a surface vessel.

FIG. 2 shows a perspective view of the transducer partially in section.

FIG. 3 shows in cross section a detail of the invention.

PREFERRED EMBODIMENT OF THE INVENTION Referring now to FIG. 1 of the drawings an acoustic transducer 10 is suspended by a yolk 11 a distance from the surface of the water. A pair of electrical leads 12 extend from the transducer interior to either monitoring or driving electronic circuitry located on a supporting vessel. The manner of attachment to the transducer and the construction of yolk must be such as to permit an axial vibration of the transducer in accordance with signals representative of acoustic energy. As shown in FIG. 2, an elongate electromechanical element 13 is disposed within the transducer to provide a dual purpose transmit-receive function. The element could be of the magnetostrictive variety which includes a core and a toroidal winding or in the form of a stack of piezoelectric ceramic discs. Due to weight considerations, methods of construction and other design criteria, the latter is preferred and consists of a stack of piezoelectric cylinders 13a having conductor discs or conductive metallic films 13b bonding adjacent ceramics together. A pair of leads electrically interconnect alternate ones of the conductor discs in a parallel relationship. Each of the pair of electrical conductors is connected to a separate one of the pair of electrical leads 12.

An axial face provided at opposite ends of the electromechanical element abuts an end plate 14 or 15 which is part of or welded onto an outer housing element 16 or 17. Each outer housing element concentrically encloses and axially extends toward the center of the electromechanical element to terminate in a horn-shaped radiating surface 18 or 19, otherwise referred to as a diaphragm member. The end plate outer housing elements and diaghragm members are preferably molded or machined from a relatively light, non-corrosive material having a low mass, such as aluminum. While what is being exemplified is a horn-shaped diaphragm member, it is readily understood that each housing element could terminate in a radially outwardly extending rounded disc or square plate with suitable seals separating the two diaphragm members to achieve substantially identical results. As shown in FIG. 3, the diaphragm members are each provided with a pair of circumferentially opposed surfaces 181: and 19a separated by an annular resilient element 20. Cross sectional area and elasticity of the resilient element must be sufiicient to permit an opposed axial displacement of the diaphragm members, e.g., a rubber-like resilient element having a T-shaped cross sectional area being in slight circumferential tension suitably meets these requirements. Outside of the resilient element, a sealing strap 21 is mounted in tension to cover the resilient element-diaphragm member junctions to seal the interior of the transducer from the transmitting medium (water). As an alternative, the sealing strap could be an annular metal rim compressing a pair of annular O-rings disposed on opposite sides of the resilient element. However, for simplicity a resilient rubber-like sealing strap is preferred.

By experimentation it has been discovered when operating the transducer as a projector of acoustic energy the stack of piezoelectric cylinders tend to tear themselves loose from the interposed conductor discs with a resultant transducer malfunction. This malfunction was remedied by providing the transducer with a means for maintaining the piezoelectric cylinders in a compressed relationship to minimize the tension portion of the transducers axial displacement cycle. In one form the transducer was provided with an axially extending rod 22 threaded on opposite ends and carrying a rubber washer 22a, a steel washer 22b, and nuts 220 at opposite ends. Both nuts are tightened to create the desired amount of tension on the stack; however, the tension exerted on the stack must not be of a magnitude which will cause the circumferentially opposed surfaces 18a and 18b to completely bridge the gap between the two diaphragm members. Another method of maintaining the piezoelectric stack in a compressed relationship is to evacuate the air from the inside of the transducer to utilize the atmospheric pressure and water pressure for the compressional force by providing at least one evacuation valve 23- adapted for connection with a distant vacuum pump.

The theory applicable to determine the optimum operational characteristics of the present invention is substantially identical to the analysis and representative equations which define the characteristics of a flat piston terminating in an infinite baflie. Here, the piezoelectric stack simultaneously displaces in opposite directions a pair of opposed flat pistons (diaphragm members 18 and 19) that are separated from its common node (infinite baffle) by a distance equal to the length of a housing element 16 or 17 and one-half of the length of the stack. The point one-half the length of the stack is the node or bafile of both of the pistons since the axial forces to or from the diaphragm members are simultaneously opposed in equal magnitude at this point. It follows that the lowest resonant frequency of the transducer is determined according to the formula:

where m=Mass of the electromechanical element and return path in grams.

At lowest frequency the water mass, an imaginary cylinder of water reactive mass, is constant. However, at higher operating frequencies the water mass increases to yield higher resonant frequencies resulting in a tracking effect between the total mass (M+m) and the resonant frequency which produces a broad band of efficient transducer operation. A complete analysis and development of the above formula can be obtained by referring to Fundamentals of Acoustics, Kinsler and Frey, 1960, John Wiley & Son, Publishers.

By being of such a compact size the instant transducer can be placed in the flooded torpedo tube of a submerged submarine.

While the invention has been described in detail for the preferred form illustrated, it will be understood that modification may be made within the scope of the invention and as defined in the claims which follow.

What is claimed is:

1. A compact transducer responsive to signals representative of acoustic energy comprising:

an electromechanical means having a pair of axial faces adapted for axial displacement in accordnce with said signals;

a pair of diaphragm members disposed in an opposed spaced relationship;

a resilient means interposed between the peripheral extremes of said diaphragm members for permitting said axial displacement;

a pair of housing elements spaced to substantially enclose said electromechanical means, each having one end secured to a separate diaphragm member and having its other end secured to a different said axial face for insuring a transfer of said signals between said diaphragm members and said electromechanical means and means for maintaining said electromechanical means in a compressed relationship being carried on said housing elements.

2. A compact transducer according to claim 1 further including:

a pair of conductors extending through said housing elements and operatively interconnecting said electromechanical means to external electronic circuitry.

3. A compact transducer according to claim 2 further including:

a yoke adapted to suspend said transducer in a surrounding medium for permitting the transfer of sonic vibrations therebetween.

4. A compact transducer according to claim 3 in which each said housing element extends toward the center of said electromechanical means in a concentric relationship.

5. A compact transducer according to claim 4 in which each said diaphragm member is configured as a radially extending horn shaped surface.

6. A compact transducer according to claim 5 in which the maintaining means is at least one valve carried on said housing elements for permitting the evacuation of air from the interior of said transducer.

7. A compact transducer according to claim 5 in which the maintaining means is an axial rod having opposite ends secured to said axial faces and concentrically transversing said electromechanical means.

5 6 8. A compact transducer according to claim 7 in which References Cited said resilient means is configured as an annular flexible UNITED STATES PATENTS ring having a T shaped cross section with the vertical portion separating said diaphragm members and further 2,832,843 4/1958 Miessnerncludmg 5 RODNEY D. BENNETT, 1a., Primary Examiner.

an annular sealing member carried in tension on the peripheral extremes of said diaphragm members and C. L. WHITHAM, Assistant Examiner. circumferentially covering said resilient means to isolate the interior of said transducer from said sur- C X-R- rounding medium. 10 34010 

